The present invention relates to a spectrum analyzer, or more in particular, to a spectrum analyzer having the function of effectively displaying the amplitude probability distribution (hereinafter sometimes referred to as the APD) of the signal field strength of a disturbance or the like, that is, the time ratio at which the level of the envelope of the signal such as a disturbance exceeds a preset threshold level, as a measure for statistically evaluating an electromagnetic environment.
In recent years, the digital communication and the digital broadcasting using such means as the portable telephone and the personal handy phone (PHS) operating in a frequency band higher than 1 GHz have extended more than ever before.
In order to protect these radio communications from disturbance, a method of measuring the disturbance and the limits thereof are under study at the International Special Committee on Radio Disturbance (CISPR).
At present, the limit of the frequency band of not higher than 1 GHz is specified primarily by the quasi peak value (QP value) of the disturbance.
This is by reason of the fact that the QP value of the disturbance is correlated with the degree of the interference in the analog communication.
It is, however, difficult to consider that the QP value of the disturbance or the peak value now being studied by CISPR, etc. as a limit in the frequency band not lower than 1 GHz is correlated with the degree of interference in the digital communication or the digital broadcasting.
For the digital communication and the digital broadcasting expected to be used more and more in the frequency band not lower than 1 GHz to be protected from the disturbance, it is desirable to determine the limit of the frequency band not lower than 1 GHz as an index correlated with the degree of interference in the digital system.
On the other hand, the communication quality of the digital communication line is expressed by a bit error rate (BER). It has been reported that the deterioration of the BER of the digital line caused by the disturbance can be estimated from the APD.
Therefore, the APD of the disturbance strength, if it can be measured accurately in simplistic fashion, is considered to provide the optimal index for evaluating the interference in the digital communication.
The apparatus for measuring the APD of the electrical signal has a long history and has hitherto been used primarily for measuring the atmospherics.
With the progress of the technologies, the APD measuring circuit has come to be configured of semiconductors and the operating speed thereof has increased. The conventional APD measuring circuit 121, however, requires as many comparators and counters as the number of the levels for determining the amplitude resolution.
For improving the amplitude resolution, therefore, it is necessary to add as many comparators and counters as the number of levels. Thus, the APD measuring apparatus of high resolution is expensive and is not suitably used widely to measure EMI.
A spectrum analyzer is also known conventionally as a disturbance field strength measuring apparatus for statistically evaluating an electromagnetic environment.
FIG. 44 is a block diagram showing a general configuration of a spectrum analyzer of this type constituting a disturbance field strength measuring apparatus for statistically evaluating the electromagnetic environment.
Specifically, as shown in FIG. 44, the spectrum analyzer comprises a front end 101, a bandpass filter (BPF) 102, a log video amplifier (LVA) 103, a peak detection circuit 104, a bottom detection circuit 105, an arithmetic unit 106 and a display unit 107.
The front end 101 is a radio wave receiving circuit including a frequency converter having a local oscillator and a mixer for producing an intermediate frequency signal (IF).
As a result, the radio disturbance or the like received through an antenna (not shown) is output from the front end 101 as an IF signal component, and then supplied to the peak detection circuit 104 and the bottom detection circuit 105 through the BPF 102 and the LVA 103.
The peak detection circuit 104 and the bottom detection circuit 105 detect the peak value and the bottom value of the envelope component of the IF signal based on the output of the LVA 103.
Specifically, let P(t) be the output signal of the LVA 103. Then, during the measurement time (Tixe2x89xa6t less than ti+T), the peak value Pp(ti) and the bottom value Pb(ti) are supplied to the arithmetic unit 106 in the form of
Pp(ti)=max P(t)
Pb(ti)=min P(t)xe2x80x83xe2x80x83(1)
In the arithmetic unit 106, the peak value Pp (ti) and the bottom value Pb (ti) for the envelope component of the disturbance or the like supplied as described above and the front end state number or a trigger signal from the front end 101 are arithmetically processed in a predetermined manner for display on the display unit 107.
FIG. 45 shows an example display on the display unit 107 of the spectrum analyzer described above.
Specifically, as shown in FIG. 45, the display unit 107 displays the peak value Pp (ti) and the bottom value Pb (ti) in the envelope component of the disturbance or the like from the arithmetic unit 106 on the frequency axis by changing the value i sequentially in the order of 0, 1, 2, . . . , Nwxe2x88x921 (Nw: resolution in frequency domain).
In this case, fc on the frequency axis designates the center frequency, f1 the start frequency and f2 the stop frequency in the measurement range (span).
As a result, the peak value Pp (ti) and the bottom value Pb (ti) of the radio disturbance expressed as
fRF(i)=f1+{(f2xe2x88x92f1)i/Nw.}xe2x80x83xe2x80x83(2)
received during the measurement time (tixe2x89xa6t less than ti+T) are read from FIG. 45 in the form analyzed on the frequency axis.
The area of P indicated by Pp (ti) greater than P greater than Pb is shown by hatching so that the amount between the peak value Pp (ti) and the bottom value Pb (ti) is easily identified.
In the disturbance measuring apparatus using the spectrum analyzer described above, however, the APD is not displayed based on the output signal P(t) of the LVA 103 during the measuring time (tixe2x89xa6t less than ti+T) constituting an important factor for statistically evaluating the electromagnetic environment, and therefore nothing can be understood of the APD.
Displaying the APD by the contour display method is another alternative conceivable. This method poses the problem, however, that as explained in detail later in comparison with the area identification display method according to this invention, the distribution with a correct threshold value contour cannot be easily identified in the case where contours of different threshold values are superposed one on the other.
An object of the present invention is to provide a spectrum analyzer having the function of effectively displaying the APD by solving the problem of the prior art described above, by employing the area identification display method using a band group having a plurality of ranges such as different color bands, for example.
Another object of the present invention is to solve the problem of the prior art described above and to provide an APD display method with a spectrum analyzer effectively capable of APD by the area identification display method using a band group having a plurality of ranges such as different color bands.
In order to achieve the above-mentioned objects, according to an embodiment of the invention, there is provided a spectrum analyzer having the APD display function, comprising:
signal receiving and processing means for receiving an input signal in accordance with desired frequency sweep information;
sampling means for sampling an output signal output from the signal receiving and processing means, based on a plurality of threshold values and outputting a plurality of output codes corresponding to sample value of the envelope of the input signal;
histogram measuring means for measuring a histogram group corresponding to the output codes produced from the sampling means;
arithmetic means for calculating an amplitude probability distribution (APD) of the output signal based on the histogram group measured by the histogram measuring means and the desired frequency sweep information and classifying the amplitude probability distribution into a group of bands having a plurality of ranges corresponding to the histogram group; and
display means for displaying the band group having a plurality of the ranges calculated by the arithmetic means as areas to be displayed in different states;
In order to achieve the objects described above, according to another aspect of the invention, there is provided an APD display method with a spectrum analyzer, comprising the steps of:
receiving an input signal in accordance with desired sweep information;
outputting a plurality of output codes corresponding to the sample values of the envelope of the input signal by sampling an output signal based on the input signal according to a plurality of threshold values;
measuring a group of histograms corresponding to the output codes;
calculating the amplitude probability distribution (APD) of the input signal based on the histogram group and the desired sweep information and classifying the APD into a group of bands having a plurality of ranges corresponding to the histogram group; and
displaying the band group having a plurality of the ranges as areas to be displayed in different states, respectively.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.